Appendix A Directions for Classroom Microcosms Much practical ecology can be taught on the high school, undergraduate, or graduate level with simple, inexpensive microcosms. Some practical suggestions follow for setting up several kinds of microcosms. Pond Type Microcosm Visit any reasonably healthy body of natural water to obtain material for a still-water microcosm. Dig up some bottom sediments, with the animals and plants that live in them, and place them in a plastic bucket. Organisms can be taken from the water column with a dip net or concentrated with a plankton net or seine. Finally, enough water should be collected to fill the planned container or containers. On the way home, avoid overcrowding of larger animals and avoid the use of metal pails. Almost any container can be used to house a microcosm, but some precautions need to be taken. New plastic or wood can sometimes bleed volatile, poisonous organic compounds into the water. New concrete will leach basic compounds, raising the pH to unacceptable levels. Aged containers of these materials are frequently satisfactory. Glass aquaria sealed with silicone rubber are the most reliable containers. Upon arriving at the location where the microcosm is to be established, the sediments and plants should first be gently transferred to the receptacle. Pond water should be added without suspending the sediments. Placing a glass or beaker on the bottom and slowly overflowing it is a good method. Lastly, the organisms should be added. A gentle stream of air bubbles will often cut down on mortality over the first few days. Water lost through evaporation should be replaced with distilled water or rain water. 445 446 Appendix A: Directions for Classroom Microcosms Prepare MIcrocosms Plastic Animals 5% of Plants Algal Ooze R Mud Types: P:R P>R PeR ~ Closed Aerator Figure A.I. Diagrammatic representation of preparation of microcosm for school use . The three relationships between photosynthesis (P) and respiration (R) are indicated along with the conditions under which they are achieved (after Odum 1960). Avoid chlorinated tap water, as it often has toxic chlorine and metal substances and unfavorable basic conditions. Three types of microcosms can be started with the same natural material. 1. One kind has more animals than can be supported by plants, as in the usual aquarium full of tropical fish . In this case, respiration exceeds photosynthesis (R > P), and oxygen comes in from the air continuously. Food for the excess animals must be supplied and carbon dioxide removed by aeration (see Chapter 8). 2. The second type is constructed with bright light, plants, and plant nutrients (the soluble " plant food" fertilizers readily available at garden Figure A.2. Closed systems supported entirely by sunlight and their overall reac­ tions. (a) Aquatic closed system; (b) terrestrial microcosm ; (c) biosphere; (d) cycle of materials between production (P) and respiratory consumption processes (R); (e) energy diagram for the systems represented in a through c (after Odum 1971). Pond Type Microcosm 447 '\\:i'kWY ' ~~~1. HoII ,:. Hoel (t) (d) Work of ",......... " ..., 11'''. planh SInIo H.t O¥t'."'t.ct.on lum . ' ~'''1 p r ______________________ "'" llJlll + COl + "10 + Minor.' . _h (( ( /,Heo' + COl + "10 + 101 '".. '........... _ ar.onic m.n., , 0, \ ~~ ______________--JJ " 448 Appendix A: Directions for Classroom Microcosms centers). Such a microcosm with have photosynthesis exceeding re­ spiration (P > R) after a couple of weeks' development in bright light. If the microcosm is under aeration, excess oxygen will be carried off in the stream of bubbles and carbon dioxide brought to the plants. Rooted aquatic biomass will visibly increase or phytoplankton will bloom and the water will turn green. 3. A third type is a balanced system where photosynthesis equals respira­ tion (P = R). This situation may result if the container is sealed. An open aquarium or other container can be sealed with plastic wrap (Figure A.l). Polyethylene film is permeable to oxygen and carbon dioxide, while Saranwrap is not. The microcosm may also be constructed in a carboy and sealed with a cork or cap. An absolute seal can be assured by pouring liquid paraffin around the cap and allowing it to harden. A balanced system (P = R) represents the entire biosphere (Figure A.2) where ultimately, the production of the plant kingdom must balance the respiration of the animals (including ourselves) and the decomposers. If humans venture very far into space in the twenty-first century, life support systems will also have to balance (see Chapter 19). Directions for Preparing a Stream-Type Microcosm A simple stream microcosm is diagrammed in Figure A.3. Support a stream bed within an aquarium above the level of water. Use a simple bubble pump to lift water from the aquarium to the head of the artificial stream. The return is screened to prevent clogging. The supports and Figure A.3. Diagram of the construction of a simple stream microcosm. Directions for Preparing a Terrestrial Microcosm 449 stream bed should be constructed of some inert material such as brick, fiberglass, ceramic roofing tiles, etc. The stream bed should be filled with small rocks, gravel, leaves, and small sticks from a natural stream, to provide seeding and a place for the attachment of organisms. The aquarium water simulates lakes above and below the run of the stream. Glass and tygon tubing are suitable for use; metals are not. Excess organic matter at the start will tend to make respiration exceed photosynthesis (R > P). Directions for Preparing a Terrestrial Microcosm Terrestrial microcosms are good means for studying the principles of ecology. Making a tiny biosphere illustrates on a small scale, nature's way of sustaining the whole biosphere. A school-room with its windows lined with microcosms is a fascinating place to watch life develop, seeing something change each day. Usually, each microcosm turns out to have something interesting and different from all the others in the room. See the example in Figure A.4. It is fun to make small microcosms and study these while also having group discussions on the events taking place in the giant Biosphere 2 in Arizona. Watch these little worlds develop, record dates on which things were done and the dates on which interesting things happened. Here are some instructions for preparing a terrestrial microcosm. 1. Chose an open or closed type. Decide if you want an open-to-air microcosm or a sealed one. The open ones are easy to observed and manipulate. By having a free exchange of air, they are automatically prevented from getting too much carbon dioxide or running out of oxygen. Figure A.4. Example of terrestrial microcosm (Metcalf, Sangha, and Kapor 1971. Reprinted with permission from Environ. Sci. Technol. 5(8) , copyright 1971 , American Chemical Society). 450 Appendix A: Directions for Classroom Microcosms However, organisms that you do not want may invade, animals may crawl out, and the microecosystem is less a model of space ship earth because it is less self-regulating. A sealed-container microcosm is forced to develop some degree of balance between the photosynthetic processes using carbon dioxide and producing oxygen, and the nighttime processes which are the reverse, using oxygen and making carbon dioxide. If well-fertilized and lighted, the plants in a sealed microcosm will fill all the space with leaves. One microcosm in a spherical container looked like a cabbage after 6 months. 2. Select a container, which can be of almost any shape, made of glass or plastic. It must hold water (not leak). At least part of it needs to be transparent so you can see what goes on. Even if you use a glass bottle, it should have a wide mouth big enough for a hand to go in and out. The cover (if there is to be a cover) can be tight enough to stop most exchange while letting a little air come and go as the air expands due to heat or shrinks as it cools down. For example, a regular aquarium container can be covered with a glass plate or plastic film held in place by a rubber band. Or, the terrarium can be tightly sealed by using a bottle with a tight screw top or rubber stopper. Pressure will increase inside when it warms up and decrease when it cools down at night. 3. Arrange light conditions. Indirect daylight (l,OOOft-candles) is best, because direct sunlight (1O,OOOft-candles) may make the ecosystem too hot and stress the animals. However, it makes an interesting microcosm for studying deserts and other hot places. Indoor light (100ft-candles) has too little energy. Microcosms in low light take a long time to develop and don't generate as much living activity. Microcosms can be made without light to illustrate what goes on in dark places such as caves or down in the soil. Such microcosms run on decomposing organic matter. For example, a half-decomposed piece of log from the woods, with all its interesting beetles, fungi, and centipedes, can be put in a rectangular aquarium-type container. Decomposer microcosms can be put anywhere in the room. 4. Decide on an ecosystem type and add some soils from that environ­ ment to the container. Think of a type of ecosystem you would like your microcosm to resemble. For example, the dead log microcosm (previous paragraph) illustrates the interesting forest floor phenomena. People can exert their creativity at this point. From the natural environment collect some typical soil materials. Interesting microcosms have been started from cattle dung, desert soil, a block of lawn grass turf, a sand bed with beach plants and animals, a patch of farm soil, peat, gravel, vermiculite, garbage, shredded plastic (to make a microcosm simulating a dump), etc.